With the recent growth of the social demand for environmental preservation, films used as the packaging materials for newspapers, magazines and food are demanded to be formed of biodegradable resins. Among biodegradable resins, polylactic acid attracts a great deal of attention because polylactic acid is widely found in nature, is almost harmless to animals and plants and to human beings and beasts, has a melting point of 140 to 175° C., has a sufficient heat resistance, has an extremely high transparency, and is a relatively inexpensive thermoplastic resin and can be produced from plant-derived raw materials.
However, sheets and films formed of polylactic acid have properties of being extremely hard and brittle as long as these sheets and films are formed only of polylactic acid, and hence it has been difficult to use these sheets and films in such fields that require flexibility, impact resistance and flex durability.
Various studies have been made, for the purpose of improving the impact resistance and flex durability of polylactic acid. For example, Patent Literature 1 discloses a composition in which polycarbonate, a transparent resin excellent in impact resistance, is mixed with polylactic acid, and which contains an organic polysilane-polycarbonate copolymer and an acrylic core-shell impact modifier. Patent Literature 2 also discloses a heat-shrinkable film made of a mixed resin containing polylactic acid and a silicone-acryl composite rubber. Patent Literature 3 further discloses a fact that the impact resistance at low temperatures is improved by adding an epoxidized natural rubber to polylactic acid.
Although the resin compositions of Patent Literatures 1 to 3 are improved in impact resistance, the resin compositions of Patent Literatures 1 to 3 are impaired with respect to the transparency, which is an advantage of polylactic acid, and hence it has been difficult to use the resin compositions of Patent Literatures 1 to 3 in the applications to films for food and industrial films for which transparency is required.
Patent Literature 4 discloses a resin composition which contains an acrylic polymer satisfactory in compatibility with polylactic acid, polylactic acid, and a graft copolymer obtained by grafting a vinyl-based monomer to a rubber polymer. Although this resin composition is satisfactory in transparency, this resin composition is poor in flexibility and texture, and also insufficient in impact resistance and flex durability.
Patent Literature 5 discloses a resin composition containing polylactic acid and a multilayered polymer. For the purpose of obtaining resin compositions excellent in impact resistance, flexibility and flex durability, in Patent Literature 5 and the related patent documents, substantially one multilayered polymer is added in a content of at least approximately 10% by mass. Unless one multilayered polymer is added in a content of approximately 10% by mass, the improvement effect of the impact resistance and flex durability is not manifested, but when the addition amount is large, disadvantageously the transparency is degraded.
Patent Literature 6 discloses a resin composition containing polylactic acid, an epoxy-modified silicone-acrylic rubber and a methyl methacrylate-butadiene-styrene copolymer rubber. By using these two specific rubber components, this resin composition is improved in impact resistance as compared to the cases where rubber components other than these rubber components are used.
However, the epoxy-modified silicone-acrylic rubber reacts with polylactic acid, and consequently the resin composition tends to be gelified; accordingly, when a resin composition containing this rubber is molded into a sheet or a film, the gelified portion is conspicuous in such molded articles, and thus disadvantageously, molded articles poor in appearance quality and transparency are exclusively obtained.
In the methyl methacrylate-butadiene-styrene copolymer rubber, the core layer of the core-shell structure is a butadiene-styrene polymer, and hence, disadvantageously the transparency of the obtained resin composition or the obtained molded articles is impaired.
As described above, the resin composition disclosed in Patent Literature 6 is improved in impact resistance, but disadvantageously, tends to be gelified to impair the excellent transparency characteristic of polylactic acid; accordingly, it has been difficult to use the resin composition of Patent Literature 6 in the applications to sheets and films for which transparency is required.